1. Field of the Invention
The invention relates generally to apparatus for pouring pre-stressed concrete, used for roadways, walls, and structural beams and supports. More specifically, the invention pertains to and apparatus and a method, for pouring pre-stressed concrete structures in situ, through the use of a transportable cable stressing frame, positioned and maintained directly over a form for the concrete structure.
2. Description of the Prior Art
Pre-stressing concrete has long been recognized as a technique to increase the tensile strength of cast concrete structures. The method generally requires that high strength wires, cables, or rods, passing through the empty mold or form for the concrete structure, are pre-stressed under high tension using a calibrated tensioning fixture. Then, the concrete is poured into the mold or form, enveloping the pre-stressed wires or cables. After the concrete has cured, the wires outside the mold are cut from the tensioning fixture, transferring the compressive forces to the concrete through the bond between the wires or cables and the concrete.
The general principles of this technique are illustrated in U.S. Pat. No. 6,773,650, issued to Longo for a Prestressed Concrete Casting Apparatus And Method. The '650 patent illustrates a pre-stressing clamshell apparatus designed to cast cementitious power poles. In this arrangement, a plurality of stationary, cable pre-tensioning devices are lined up at a production facility. The movable clamshell mold surrounds each pre-tensioning fixture while the concrete is poured and allowed to set. Then, the mold is opened and lifted up, and then moved along to the adjacent fixture, where the process is repeated.
An Apparatus For Making Prestressed Structural Members is disclosed in U.S. Pat. No. 3,049,786, granted to Jones. This apparatus uses a cable pre-stressing fixture like that shown in the '650 patent, but relies upon a movable mold member 60. As concrete is poured into the mold member, the mold member is slid along the fixture until the entire poured structure is formed over the cables.
In U.S. Pat. No. 3,260,024, issued to Greulich, a Prestressed Girder is shown. This reference suggests that the girder can be constructed either at the prestressing plant or at the building site, using an apparatus such as that depicted in FIG. 1. A horizontal beam 3 includes jacks 1 mounted on opposing anchor members 2. The concrete pour is made over the cables and the underlying beam. There is no particular adaptation or suggestion how this apparatus might be used in the field, for example at a building site, other than simply transporting the same apparatus that is used at the prestressing plant to the building site.
Basically the same methods discussed above are used to manufacture pre-stressed concrete slabs or roadway segments. These concrete structures are used for new road construction, or for purposes of road repair. For example, in making a new freeway, or in repairing damaged portions of roadway, a concrete slab or roadway segment is manufactured at an off-site facility, using a cable pre-stressing apparatus and a form or mold arrangement associated with that apparatus. After the concrete is poured and cured, the slab is transported by truck or rail to the roadway site for installation. In preparing a bed within which the new slab is to rest, every effort is made to match the inclination, orientation, and depth of the bed with that of the new slab, so that a smooth roadway transition can be made between adjacent slabs. Notwithstanding these efforts, it is very difficult to effect a perfect match between the bed and the slab, and surface anomalies and gaps do occur between adjacent slabs.
Similarly, it is conventional that pre-stressed walls, beams, posts, and other concrete structures are manufactured at a production facility, where permanent fixtures are located for pre-stressing cables and forms are provided to determine the size and configuration of the concrete structures. As with the roadway slabs, after pouring and curing, these concrete structures must also be transported to a remote building site, offloaded, and assembled or arranged as required.